The present invention relates to a magnetic transducer for recording and reproducing a high frequency signal on a magnetic medium and, more particularly, to an arrangement of an improved magnetic transducer.
The magnetic transducer of the above described type is used, e.g., in a video tape recorder. As a core material for the magnetic transducer, ferrite having a high ohmic resistance has been widely used. However, in a case where the magnetic medium is a high coercive tape, such as a so-called metal tape, the ferrite core may not sufficiently record or reproduce the signal because the saturation point of magnetization, or a saturation flux density (B.sub.s), of the ferrite is comparatively low.
In consideration of the above, a material having a very large saturation flux density, such as sendust (Fe--Si--Al alloy), has recently been taking the place of ferrite as the core material. Furthermore, in order to record and reproduce a high frequency signal, the thickness of the sendust core is made very thin, such as several 10 s microns. Since such a thin sendust core is easy to break and easy to be worn out by the abrasion between core and running tape, it is reinforced by a pair of reinforcing cores which sandwich the sendust core therebetween. Each of the reinforcing cores is defined by a ferrite plate and a glass mounted on one side face of the ferrite plate. An example of such an arrangement is disclosed in U.S. patent application Ser. No. 231,321 filed Feb. 4, 1981 or in corresponding German Patent Application No. P 31 03 767.4 filed Feb. 4, 1981, and laid open to public inspection as Offenlegungsschrift No. 31 03 767 on Dec. 24, 1981, both of which are assigned to the same assignee as the present application.
When in use, a side face of the sendust core where a head gap is formed is made in flush with a side face of the glass, so that the magnetic tape may slidingly move past the face of the magnetic transducer. According to the prior art, a material for the glass is chosen in consideration of the hardness of the sendust core based on the Vicker's hardness test. More particularly, in the prior art magnetic transducer, the glass is so selected as to have a hardness much harder than the sendust core, and this selection is based on such an assumption that the glass members provided on both sides of the sendust core prevent wear down of the sendust core. According to the tests carried out by the present inventors, however, it has been found that, after a long period of operation, such a prior art arrangement has acceptable recording and reproducing abilities in a few cases, but poor in many other cases, particularly with respect to high frequency signals. According to a further investigation, it has been found that, after a long period of operation, some test pieces of magnetic transducer has its sendust core projecting above from, or in flush with, the level of the face of the glass members in 0.1 micron order, and many other test pieces had its sendust core worn down below said level in 0.1 micron order. The ones that showed poor recording and reproducing abilities are the latter ones, which disadvantageously forms a space between the face of the sendust core and the running tape. This can be understood from the spacing loss characteristic, as shown in FIG. 12, known to those skilled in the art. In the graph of FIG. 12, abscissa and ordinate represent d/.lambda. (d is the distance of the space between sendust face and running tape, and .lambda. is wavelength of recorded signal) and spacing loss in dB, respectively. The graph of FIG. 12 shows that, as the distance d of the space increases, or as the wavelength becomes shorter, the spacing loss becomes more and more perceptible, resulting in poor recording and reproducing abilities.
From these tests, it has been concluded that the abrasion of the magnetic transducer has no relationship with the hardness of the material forming the transducer.
During the tests, it has also been found that the conventional magnetic transducer has such a disadvantage that the glass members mounted on the ferrite plate are easily dropped off, particularly after a long period of operation.